text: Let  be oil relative permeability, and let  be water relative permeability. There are two ways of scaling phase permeability (i.e. effective permeability of the phase). If we scale phase permeability w.r.t. absolute water permeability (i.e. ), we get an endpoint parameter for both oil and water relative permeability. If we scale phase permeability w.r.t. oil permeability with irreducible water saturation present,  
   endpoint is one, and we are left with only the  endpoint parameter. In order to satisfy both options in the mathematical model, it is common to use two endpoint symbols in the model for two-phase relative permeability.
  The endpoints / endpoint parameters of oil and water relative permeabilities are
  These symbols have their merits and limits. The symbol  emphasize that it represents the top point of . It occurs at irreducible water saturation, and it is the largest value of  that can occur for initial water saturation. The competing endpoint symbol  occurs in imbibition flow in oil-gas systems.  If the permeability basis is oil with irreducible water present, then . The symbol  emphasizes that it is occurring at the residual oil saturation. An alternative symbol to  is  which emphasizes that the reference permeability is oil permeability with irreducible water  present.
  The oil and water relative permeability models are then written as
  The functions  and  are called normalised relative permeabilities or shape functions for  oil and water, respectively. The endpoint parameters  and  (which is a simplification of ) are physical properties that are obtained either before or together with the optimization of shape parameters present in the shape functions.
  There are often many symbols in articles that discuss relative permeability models and modelling. A number of busy core analysts, reservoir engineers and scientists often skip using tedious and time-consuming subscripts, and write e.g. Krow instead of  or  or krow or oil relative permeability. A variety of symbols are therefore to be expected, and accepted as long as they are explained or defined.
  The effects that slip or no-slip boundary conditions in pore flow have on endpoint parameters, are discussed by Berg et alios.